PTT, Dave Un­win

Pilot - - CONTENTS - DAVE UN­WIN Pilot’s Flight Test Editor op­er­ates a Jodel D.9 from a farm strip and has logged stick-time on ev­ery­thing from ul­tra­lights to fast jets

Keep­ing track of mea­sure­ments and def­i­ni­tions in log­books and else­where

While fill­ing in my Jodel’s log­books the other day I no­ticed that the air­frame had just rolled over 1,000 hours, which is a bit of a mile­stone. By a cos­mic co­in­ci­dence, although nei­ther my power nor glider logs ap­peared to be sig­nif­i­cant, when I added the two to­gether for my to­tal to­tal time (as I do about twice a year) it was also a noteworthy num­ber, and this got me think­ing about the log­book and its ori­gins. As the name sug­gests, orig­i­nally the log­book was used to record read­ings from the ‘chip log’, which was for cen­turies the pri­mary method of de­ter­min­ing a ship’s speed through the water and ab­so­lutely fun­da­men­tal to nav­i­ga­tion. Ba­si­cally, the chip log was a form of drogue (ini­tially it re­ally was just a log) that was con­nected to a thing called a log-line and dropped over the stern. The log-line fea­tured reg­u­larly spaced knots, and while one sailor mon­i­tored a sand­glass another counted the knots as they passed over the stern rail. It is a com­mon mis­con­cep­tion that these knots were 100ft apart, prob­a­bly be­cause an Im­pe­rial nau­ti­cal mile, some­times called an Ad­mi­ralty mile, or more ac­cu­rately, an Ad­mi­ralty mea­sured mile, is 6,080ft, and 100 feet times 60 min­utes is 6,000ft. This would ap­pear to make ‘one knot’ a hun­dred feet a minute, or one nau­ti­cal mile an hour. In fact, most log-lines had the knots spaced at in­ter­vals of eight fath­oms (48ft) and were used in con­junc­tion with a half-minute sand­glass,

It seems more likely that once the Dutch sci­en­tist Snell had cal­cu­lated the world’s cir­cum­fer­ence as 24,024 miles in 1617, Bri­tish math­e­ma­ti­cian Ed­mund Gunter proposed that us­ing lines of lat­i­tude could be used to mea­sure dis­tance, and sug­gested that a nau­ti­cal mile should be one minute (one six­ti­eth) of one de­gree of lat­i­tude. As one de­gree is 1/360th of a cir­cle, one minute of arc is 1/21600 of a cir­cle, mak­ing a nau­ti­cal mile 6,080 feet, which is the length of one minute of arc at 48 de­grees lat­i­tude (as the Earth is not a per­fect sphere , a minute of lat­i­tude is not con­stant, and 48 was deemed a good com­pro­mise). Of course, as aviation has evolved we’ve grown used to a mish-mash of units, which is why although the length of the run­way is in me­tres, the height of the trees that we’re try­ing to avoid at the far end is given in feet, and why an air­craft fly­ing VFR in con­trolled airspace must re­main 1,500m hor­i­zon­tally and 1,000ft ver­ti­cally from cloud, and in a flight vis­i­bil­ity of at least 5km at all times!

For many years west­ern ASIS were often in mph and flight-plan­ning was done us­ing statute miles. There is an in­ter­est­ing al­beit pos­si­bly apoc­ryphal story that dur­ing WWII some RAF fight­ers were launched from a Royal Navy car­rier to re­in­force Malta, and that although the Navy had flight-planned us­ing nau­ti­cal miles, dur­ing the pre-launch briefing this wasn’t made clear and the RAF types just as­sumed the dis­tance was in statute miles. On a 600nm flight the dif­fer­ence is not in­signif­i­cant, and the last ninety miles would’ve cer­tainly con­cen­trated the mind!

Per­son­ally I pre­fer an ASI in knots, par­tic­u­larly when glid­ing as the var­i­ome­ter is also cal­i­brated in knots, and with the al­time­ter in feet this al­lows you to make quite rapid fi­nal glide cal­cu­la­tions. Not that any­one does any more of course, it’s all elec­tronic - but it’s how I was taught. I’m not a fan of the met­ric al­time­ter (I find the scale ex­pan­sion poor, as a sin­gle sweep of the dial is 3,280ft) but will al­low that — oc­ca­sion­ally the met­ric ASI has its uses — most mem­o­rably when I flew Peter Hol­loway’s mag­nif­i­cent Fieseler Storch sev­eral years ago. If you’ve ever stood next to a Storch you’ll know that — stand­ing over three me­tres tall with a four­teen-me­tre wing­span and a MAUW of around 1,300kg — it’s quite a big ma­chine. Con­se­quently, when Peter said “Use eighty coming around the cor­ner, and no less than seventy over the fence,” it sounded rea­son­able. If the ASI had been in knots, and he’d said “Use 43 coming around the cor­ner and no less than 37 over the fence,” it would’ve sounded a lot less be­liev­able!

On a dif­fer­ent tack...

It’s al­ways been in­cum­bent upon all avi­a­tors to keep their log­book cor­rectly, and with the re­cent furore sur­round­ing the well-known avi­a­trix Tracey Cur­tis-tay­lor now cross­ing over from in­ter­net fo­rums to the main­stream media, I found my­self won­der­ing if the def­i­ni­tion of ‘solo’ had some­how changed from the one I was fa­mil­iar with. From what I can gather, Ms Cur­tis-tay­lor has gath­ered an im­pres­sive clutch of awards, medals and Hon­orary Doc­tor­ates while ‘repli­cat­ing’ the solo flights of some amaz­ing fe­male avi­a­tors from the past in a beau­ti­ful Stear­man, yet peo­ple per­sist in post­ing pic­tures of her ar­riv­ing at var­i­ous des­ti­na­tions with the front cock­pit of said Stear­man clearly oc­cu­pied. As some of the more — shall we say ‘colour­ful’ — air­men of my ac­quain­tance might ask, WTF is that all about? It’s cer­tainly a sub­ject that has piqued my in­ter­est.

As aviation has evolved we’ve grown used to a mish-mash of units I found my­self won­der­ing if the def­i­ni­tion of ‘solo’ had changed...

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